Increased platelet reactivity contributes to a greater risk of thrombosis, the proximate cause of heart attack and stroke. Given the negative health effects of thrombosis, assays for platelet reactivity should be useful for identifying individuals who are at risk of thrombosis, and in selecting appropriate therapeutic regimens. However, current assays of platelet reactivity have not demonstrated that capacity and are sensitive to medications and other therapies which are in common use. Therefore, novel assays for platelet reactivity that can guide therapy and are insensitive to common therapies and medications are needed.